I was fixing an SMPS, and then this happened!

[Drake122]

Hey guys!

I work as a computer tech and I'm trying to get into chip-level repair and such. I was working on a Switch Mode Power-supply that had some issues, I was probin' around the parts (watched some vids, so I can identify them  ) and as I was probing voltage on a high-voltage transistor to see if its working, It blew. Arc over the probes, the fuse saved my ass that day. It was pretty scarry. I want to ask you guys, what might have happened. I ruled out the clearance between the probes, I didn't bring them too close, they have to be really close for an arc to happen. I guess I shouldn't test transistors in circtuit like that? 

ps.: I can post some pics about the parts and area I was working on tomorrow, when I get to work. I don't really have the schematics for that PSU, pretty cheap stuff

[danadak]

Possibly ground related issues.

Here is a discussion (and ap notes)


https://www.eevblog.com/forum/beginners/differential-probes-or-'float-the-scope'/msg1427668/#msg1427668


Regards, Dana.

[madires]

As a beginner please disconnect the SMPSU from mains and discharge the main filter cap before checking components. Probing voltages in the primary side requires some experience and knowledge, also the right tools.

[Mechatrommer]

maybe he got his lesson as a beginner. probing hot line with hand is bad idea as hand can moved around and moving probe into distance initiating arcs. next time turn off the mains. use probe clip and make sure attached firmly to circuit to be probed and then turn on the mains. never connect ground clip to anything (for non isolated scope).

[Drake122]

Okay, I get it now. Don't probe live wires unless I know what to look for 
As for SMPS repair, what usually goes bad, after the most obvious thing (capacitors)?

[madires]

Switching MOSFET, switching controller, controller's filter cap, feedback (opto coupler), Schottky diodes in the secondary side...